The greatest resource of wave energy is at off-shore sites with open exposure to the prevailing winds and at water depths greater than half a wave-length of the prevailing wave conditions. In practice, this means that the best ocean wave energy is at depths of the order of 100 metres or more.
Many wave energy conversion technologies include one or more bodies that oscillate in one or more modes as a result of excitation forces induced by the incident wave field and thus absorb energy as kinetic and potential energy. Necessarily such oscillating systems must themselves be large and massive and capable of reacting against another large and massive body or the Earth via a power take-off if significant energy is to be converted to useful power. The optimum size of such an oscillating system is defined by the prevailing wave-length, and economics. To be commercially viable at utility scale, a wave energy converter must operate effectively in large arrays in the most energetic offshore environments.
Axi-symmetric wave energy point absorbers are well suited for offshore deployment in large arrays; most of these are heaving buoy point absorbers which oscillate or heave in a vertical mode. These may typically react against the sea-bed via taut moorings or a spar, or be self-reacting by comprising more than one large oscillating body, or react against an internal mass, or an external inertial mass or plate. Reacting against the sea-bed requires that the spar or taut mooring is sufficiently secured or anchored to the sea-bed so that it will withstand the forces that correspond to power outputs of the order of a megawatt or more.
Such anchoring systems are expensive to install and maintain in deep water. Alternatives to such an approach include the provision of two or more large bodies reacting with each other through a power take-off. Such oscillating systems pose major engineering challenges to ensure that they can maintain alignment and continue to function for many million cycles and withstand stormy conditions. This essential need to react against something has posed a significant challenge to the development of practical, sea-worthy and cost-effective oscillating wave energy absorbers, and in particular those devices that may be classed as self-reacting.
A further essential requirement of a heaving buoy point absorber is that its natural period of oscillation in the vertical mode or heave should be capable of matching that of the incident wave if maximum energy is to be absorbed, a condition known as resonance.
A heaving buoy point absorber tends to have a well-defined natural period in heave and as a result responds best and absorbs energy efficiently from a narrow band of the total energy distribution. Thus it is advantageous to be able to adjust the device's natural period. A number of compromise solutions have been proposed, such as latching, (where the oscillation is held or ‘latched’ momentarily to simulate a longer natural period) or increased damping so that the absorber's response is spread, but at the cost of reducing the peak.
Oscillating water columns (OWC) comprise a well-established class of wave energy converters and this technology has been applied in both on-shore and offshore systems. The water column within an OWC is activated by the incident waves. As with other oscillating wave energy absorbers, maximum energy absorption occurs when the natural oscillation of the water column, with the air trapped above it, is in step with the incoming wave train. This condition is closely defined by the geometry of the chamber that encloses the oscillating water column and the air above it.
A further well known implementation is to include an OWC as an integral part of a heaving spar buoy. A spar buoy is one where the width is small in comparison to the draught. The width of any point absorber should be small with respect to the prevailing wave length, not much more than 20 m in diameter for a typical ocean site. For periods in the range 8 to 14 seconds, an OWC of 10 metre radius would require water column lengths from 8 to 40 metres to ensure that the interior water column might resonate. Such a range of draughts is difficult to implement in practice. Furthermore, the available wave energy is much diminished at depths approaching 40 metres. An additional problem is the need to ensure that the oscillations of the heaving spar buoy and the enclosed OWC are out of phase otherwise it is difficult to recover power as there is no relative movement between the two oscillating systems.
Thus the development of a commercially viable point absorber wave energy converter has been hampered by a number of significant challenges, for example:                Reacting against the sea-bed is costly to install and to maintain in deep water offshore.        Self-reacting resonant heaving buoy point absorbers have heretofore required at least two massive bodies articulated via the power take-off, requiring careful alignment and end-stop control.        Articulated devices tend to be less sea-worthy, more prone to failure, and more costly to manufacture and to maintain.        The performance of a floating resonant OWC point absorber is constrained by the dimensions of the water column; adjusting this to suit varying wave conditions involves complications or, alternatively, a multiplicity of different water columns.        A point absorber that incorporates an OWC in a surface-piercing buoy may have difficulty in ensuring an adequate phase difference between the oscillation of the internal water surface and the heaving buoy for effective power recovery across a typical wave energy distribution.        
There is therefore a need for a wave energy converter which addresses at least some of the drawbacks of the prior art.